Constant Variables

The design of HVAC pumping systems has been greatly affected by the advent of variable-frequency drives (VFDs). These devices have created a multitude of design options for engineers and arguably helped increase energy efficiency. In this month's M/E Roundtable, our panel of HVAC design experts discuss the effects of VFDs, some design considerations and the shape of things to come.

By Jeromie Winsor, Web Editor

08/01/2002

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The design of HVAC pumping systems has been greatly affected by the advent of variable-frequency drives (VFDs). These devices have created a multitude of design options for engineers and arguably helped increase energy efficiency. In this month's M/E Roundtable, our panel of HVAC design experts discuss the effects of VFDs, some design considerations and the shape of things to come.

CONSULTING-SPECIFYING ENGINEER (CSE): How much has the integration of VFDs changed the design of HVAC pumping systems?

RISHEL : It has profoundly changed HVAC design. In pumping, we have eliminated a number of devices and circuits that were provided to overcome the overpressure caused by constant speed pumps.

ALEXANDER : Before VFDs, system designs were limited. Some incorporated multiple pumps or mechanical means of changing speeds, and these options could prove costly or unreliable.

SMIERCIAK : In the past, the only options were staging multiple pumps, riding the pump curve or using a constant-flow system. Properly applied VFD pumping systems save pumping energy compared to each of these alternatives.

KOKAYKO : Recently, the falling cost of drives and rising cost of utilities are leading to wider integration of VFDs into small- and medium-sized pumping systems. In the past, higher drive costs limited their use to larger systems to achieve reasonable payback.

On the large systems we're seeing an increased consideration of variable-speed pumping of the primary chilled-water loop.

CSE: What do you see as the primary benefits of these systems?

SMIERCIAK : The primary benefit of incorporating VFDs into a variable-flow pumping system is the energy savings compared to a constant-speed, constant-flow system or constant-speed, variable-flow system.

As a general comparison, if the system loads only required 50% of design flow, the constant-speed, constant-flow pumping system would operate at 100% of design horsepower (hp). The constant-speed, variable-flow pumping system would operate at approximately 70% of design hp, and the variable-speed, variable-flow system would operate at approximately 25% of design hp. This is based on a typical pumping-system pressure profile consisting of a 75% variable head loss and a 25% constant head differential component.

RISHEL : In addition to energy efficiency, other benefits are simplified piping circuits and a reduction in pump maintenance, as we no longer run the pumps up their curves to points of high radial thrust.

KOKAYKO : Call me old-fashioned, but I personally prefer to see constant primary flow through chillers. I feel that variable-speed pumping should be reserved for the secondary transport piping loop, and possibly even extended to a tertiary piping loop on a central campus plant or a large central facility. That sort of piping arrangement is a nice trade-off of pumping efficiency and failsafe operation, and with proper design, pumping transport costs can be reduced and plant efficiency increased.

Because facility maintenance budgets and staffs have been shrinking, I feel that complex or sensitive controls can be a liability over the long haul. I don't feel that it's wise to always try to wring every last nickel out of the operating costs, and in doing so, inadvertently saddle a client with ongoing operating problems.

CSE: How common are these variable-speed, variable-flow pumping systems?

SMIERCIAK: The number has dramatically increased in the last decade because of the escalation of electric utility rates, the reduction in costs for VFDs, the increased reliability of the electronics and a growing awareness of the impact of pump energy on the cost of plant operation.

RISHEL: Almost all HVAC pumping systems are variable speed where the pump motors are 10 hp or larger. The only exceptions are constant cooling loads such as computer centers, where there is very little change in cooling load.

SMIERCIAK: The use of VFDs will continue to increase as states begin to adopt new energy codes. The ASHRAE 90.1-2001, Energy Standard for Buildings Except Low-rise Residential Buildings , already requires that pumping systems with modulating valves shall be designed as a variable-flow system. Actually, the standard goes even further and requires that individual pumps exceeding 50 hp and 100 ft. head incorporate controls such that at 50% of design flow, the system will not utilize more than 30% of design hp.

CSE: Should a system using VFDs always be considered?

KOKAYKO : Variable flow should be considered if the load on the system is variable. That's not meant to be a flip response, but it is surprising how many designs stick VFDs on the pumps for no apparent reason. If the plant load is constant, or fairly constant year-round, variable flow probably doesn't make much sense. Data centers were previously mentioned, and are a great example of a building type that has negligible external gains relative to its internal gains. Some buildings employing highly efficient curtainwall systems or super-insulated wall constructions may also fall into that category.

SMIERCIAK : Constant-speed, constant-flow and constant-speed, variable-flow pumping systems are still viable and recommended for constant load applications, smaller systems with minimum distribution and terminal devices, or systems that have a very small variable head-loss component.

ALEXANDER : Yes, if an application involved a steady load, unchanging with time, then it would make sense to have a constant flow distribution system. Rarely is this the case. Nevertheless, there are situations where specific equipment cannot tolerate flow rate fluctuations. For example, excessive flow rates can erode pipe and tubing.

CSE: Is variable-flow pumping often misapplied?

KOKAYKO : Definitely. Using it on a building type or load profile such as I described before would be an example. But I've actually seen variable-speed pumping touted as a means of energy savings when the entire system was controlled using three-way valves. What this was supposed to resolve, without a complete overhaul of the piping and control system, was beyond me.

RISHEL : The only misapplication that I have seen is using variable speed on low-head jobs where it was not justifiable.

ALEXANDER : I also find that variable speed is rarely misapplied, but an example is when constant-flow valves are used with variable-flow pumps. When this is done, the system won't work properly. For example, as Mike mentioned, three-way control valves will thwart the design intent of a variable-flow system because there isn't a change in the flow rate.

CSE: So when designing for variable-flow, is a primary-only system the best? When does a primary-secondary system work better?

RISHEL : Primary-secondary systems are seldom needed now because variable flow is allowed in most chillers and boilers. Some firetube boilers should still have boiler pumps to maintain steady flow.

KOKAYKO : Actually, my personal preference is for primary-secondary systems. Certainly, large primary-only systems offer their own distinct advantages, but I just feel that in many cases the savings is not enough to warrant the possibility of operational problems.

ALEXANDER : Every system type needs to be evaluated. A simple system with few branches can be a good candidate for primary-only distribution. Complex systems benefit from secondary pumping, which reduces installed horsepower and energy consumption. If a system has several branches with different pressure drops, the most effective way to deal with it is to provide secondary pumps for the individual loop flow rate and pressure drop. In contrast, a primary-only system would have to have a pump sized for the worst case flow and pressure drop.

SMIERCIAK : I agree. The primary-only pumping system is not necessarily the best application for all projects. The primary-secondary pumping system lends itself very well to large-scale applications where you may experience multiple minimum pressure requirements between user groups.

CSE: How do you figure out the optimal staging of pumps? Is there a particular software or methodology that you use?

SMIERCIAK : Here is a generic control strategy for staging pumps:

The speed of the lead pump is controlled to maintain the minimum required differential pressure at or near the most remote user.

Whenever the lead pump reaches or exceeds 95% (adjustable) of its speed the lag pump is energized. Whenever the lead pump reaches 30% (adjustable) of its speed the lag pump is de-energized.

The speed of the lag pump is controlled to match the speed of the lead pump. This approach synchronizes pump operation and avoids the potential for the control loops to fight against one another. In the event of lead pump failure, the lag pump becomes the lead pump and reverts to differential pressure control.

RISHEL : I use kW input for the programming of multiple-pump systems. Wire-to-water efficiency can also be used. Using a percent of maximum speed is usually wasteful energywise.

KOKAYKO : One particular control method I'm very interested in is the use of a floating setpoint for pressure control of transport pumps.

Often the transport pumps operate to maintain a fixed pressure at some point in the piping system. Instead of using a fixed pressure for this control, I prefer to have the setpoint be continuously reset to maintain flow at the most critical tertiary loop in the system. The most critical loop is determined by using fluid flow meters and valve pilot positioners on each tertiary loop.

By maintaining the valves more open by using a floating pressure setpoint, transport pump head is kept at its minimum value. As a result, power consumption is reduced.

CSE: For a variable-flow system, what is the best method of determining and controlling the proper pump speed?

SMIERCIAK : Typically, the pump speed should be controlled to maintain the minimum required differential pressure at or near the most remote user or the user requiring the largest pressure drop. It is important during the start-up and commissioning phase to set the differential pressure setpoint as low as possible; the lower the setpoint, the higher the potential pump energy savings.

RISHEL : Temperature should seldom be used as a determining factor. For most HVAC systems, the remote differential pressure transmitter is the preferred means of control.

KOKAYKO : But in a primary-secondary (or tertiary) central plant arrangement, I think you need to use both temperature and pressure to determine pump speeds. The primary-loop pumps would circulate water through the system's prime movers at a constant volume. Staging would be based upon temperature and/or flow-through in the primary/secondary bridge piping.

The secondary, or transport pumps would supply and return water to either the load or tertiary pumps based upon pressure at the most remote device. The tertiary pumps, or load control valves, in the case of primary-secondary pumping, would operate to maintain a target temperature differential between the water taken from and returned to the secondary loop.

CSE: What are some current or future developments in VFDs for these systems?

ALEXANDER : One of the issues that I would like to see refined is the bearing-failure problem. Apparently, some drive and motor combinations cause stray currents to arc through the bearing. This has resulted in early failure of the bearings. It is something that needs to be researched and incorporated into equipment and system design.

RISHEL : I think that the reduction in cost and size have been most significant. Also, the installation of running limits has helped reduce the size of pump drives and motors.

KOKAYKO : Years ago, specifying a drive without a bypass was considered foolhardy. Due to the high reliability and relatively low drive costs, now we typically specify drive bypasses on larger drives or mission critical applications.

Also, advances in open-control protocols and better processing capabilities allow more seamless integration of drives into virtually every modern digital control system.